程序代码：

#include <fcntl.h>

int dup2(int fd1, int fd2) {
    
    if( fd1 != fd2 ) {

        /* check to make sure that fd1 is a valid open
         * file descriptor
         */
        if ( fcntl( fd1, F_GETFL) < 0 )

            return -1;

        /* check to see if fd2 is already open;
         * if so, close it
         */
        if( fcntl( fd2, F_GETFL) >= 0 )

            close ( fd2 );

        if(fcntl ( fd1, F_DUPFD, fd2 ) < 0)

            return -1;
    }

    return fd2;
}